In general, there are two methods of feeding print media (e.g. paper) past a printhead in a printer. Desktop printers typically feed individual sheets of paper from a stack of paper held in a paper tray. Individual sheets of paper are taken from the top of the stack on demand and fed past the printhead.
In large-scale wide format printing, the print media is typically a continuous web. The web of print media is supplied as, for example, a roll of paper, which can be fitted onto a supply spool in the printer. During printing, the web is fed from the supply spool, past a printhead and onto a take-up spool. Usually, a drive roller system, comprised of a pair of grippingly engaged rollers, is positioned between the supply spool and the take-up spool. The drive roller system feeds the web past the printhead on demand.
In all commercially available wide format printers, a scanning printhead is employed to deposit ink on the web of print media. In such printers, the web must be stationary as the printhead traverses across the web. After each scan of the printhead, the web moves forward and the printhead scans across again, depositing the next line of an image.
U.S. Pat. No. 6,672,706 (Silverbrook) describes a wide format pagewidth inkjet printer. In this wide format pagewidth printer, the web is continuously fed past a pagewidth printhead. The pagewidth printhead makes high-speed wide format printing possible by “printing-on-the-fly”—that is, continuously feeding a web and simultaneously printing without the web having to be stationary at any stage.
It will be appreciated that, in order to achieve “printing-on-the-fly”, it is important that the delivery of the media is finely controlled to achieve consistent print quality. Any variation in web speed or web tension would result in a deterioration in print quality in the form of, for example, a distorted printed image. A constant web speed and web tension requires not only a reliable feed motor system, but also secure attachment of the web between the supply spool and the take-up spool. It is particularly important to avoid slippage of the supply spool when the web is under tension, otherwise a loss of tension and poor print quality may result.
In general, print media for wide format printers are supplied as rolled webs having a hollow cardboard core. These rolls of print media are usually loaded into wide format printers by threading a spindle (or axle) through the hollow cardboard core. The spindle is then loaded into the printer by connecting it to suitable end-mountings, which cooperate with the spindle to form a supply spool. The roll of print media is free to rotate allowing the web to be fed to drive rollers. The drive rollers draw print media from the supply spool, feed it past a printhead, and then onto a take-up spool driven by a take-up motor.
When “printing-on-the-fly” at high speeds, it is especially important to keep the web of print media under tension at all times. Any sagging between the drive rollers and the supply spool may result in crumpling of the print media when the printer is being run at high speeds. Accordingly, in continuous high-speed wide format printing, the supply spool is usually connected to a braking motor, which opposes the natural rotation of the supply spool. This natural rotation of the supply spool is a result of the drive rollers drawing the web from the supply spool. The braking motor provides a counter-rotational force, which generates tension in the web between the supply spool and the drive rollers. A problem with the traditional supply spool arrangement described above is that the cardboard core of the rolled web tends to slip over the spindle when a braking force is applied to the spindle, thereby diminishing the effect of any braking force applied.
It would be desirable to provide a spool, such as a supply spool for a printer, which minimizes slippage of the rolled web of print media relative to a spindle on which it is mounted. It would be particularly desirable to maximize transmission of a force from a braking motor to a roll of print media so that tension in the web can be generated and accurately controlled.